metamorphic

Banded gneiss

Gneiss, likely quartz–feldspar–biotite gneiss

AI-generated identification · may be incorrect

Typically medium- to coarse-grained, with alternating light quartz/feldspar and darker biotite or amphibole bands; hardness about 6–7, nonmetallic luster, and weak or absent cleavage. The specimen shows strong compositional banding and foliation, with fractures crossing the layers.

Identified August 19, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or other felsic rock underwent intense regional metamorphism, commonly at amphibolite- to granulite-facies temperatures and pressures. Its exact age cannot be determined visually; gneiss ranges from Archean to younger continental metamorphic belts.

Uses & applications

Used as crushed stone, dimension stone, landscaping rock, and occasionally polished architectural stone; attractive pieces are collected but are generally not gemstone material.

Geological facts

Gneissic banding results from mineral segregation during deformation and recrystallization, not ordinary sedimentary layering. It commonly records deep burial and mountain-building events.

Field identification & locations

Identify it by alternating light and dark mineral bands, coarse interlocking grains, and a hard, crystalline texture; unlike schist, it splits poorly and has less abundant aligned mica. Common in Precambrian shields and high-grade mountain belts.